Diabetes is a medical condition based on metabolic disorders caused by lack of insulin, which is one of the hormones produced by the pancreas. A diabetic patient has to use a method of injecting insulin into his/her body as one of the aggressive methods. An insulin injection device may be used to appropriately inject insulin into the body according to a change in a patient's blood glucose.
A medicinal fluid injection device such as an insulin injection device may be semi-permanently used while constantly replenishing a medicinal fluid therein. In such a case, foreign materials (e.g., air), other than the medicinal fluid, need to be prevented from being injected through an injection needle into the body or the medicinal fluid injection device needs to be prevented from being contaminated.